1. Field of the Invention
The present invention relates to improved methods and compositions for consolidating proppant in subterranean fractures.
2. Description of the Prior Art
Hydrocarbon-producing wells are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a producing zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Particulate solids, e.g., graded sand, for propping the fractures, commonly referred to in the art as “proppant,” are suspended in a portion of the fracturing fluid so that the particulate solids are deposited in the fractures when the fracturing fluid reverts to a thin fluid to be returned to the surface. The proppant deposited in the fractures functions to prevent the fractures from fully closing so that conductive channels are formed through which produced hydrocarbons can flow.
In order to prevent the subsequent flow-back of proppant and other particulates with the produced fluids, a portion of the proppant introduced into the fractures has heretofore been coated with a hardenable resin composition that is caused to harden and consolidate the proppant particles in the fracture. Typically, the hardenable resin composition-coated proppant is deposited in the fracture after a large quantity of uncoated proppant has been deposited therein. That is, the last portion of the proppant deposited in each fracture, referred to in the art as the “tail-end” portion, is coated with the hardenable resin composition. When the fracturing fluid, which is the carrier fluid for the proppant, is broken and reverts to a thin fluid as described above, the hardenable resin-coated proppant is deposited in the fractures and the fractures close on the proppant. The partially closed fractures apply pressure on the hardenable resin-coated proppant particles whereby the particles are forced into contact with each other while the resin composition hardens. It has heretofore been thought that the hardening of the resin composition under pressure brings about the consolidation of the resin-coated proppant particles into a hard permeable mass having sufficient compressive and tensile strength to prevent unconsolidated proppant and formation sand from flowing out of the fractures with produced fluids. However, it has been found that as proppant without a hardenable resin composition coating thereon is carried into the fracture by the fracturing fluid, some of the proppant is continuously deposited in the bottom of the fracture adjacent to the wellbore. This unconsolidated accumulation of non-resin-coated proppant remains in the fracture adjacent to the wellbore and when the hardenable resin-coated proppant enters the fracture at the end of the treatment, it does not displace the uncoated proppant already deposited at the bottom of the fracture. Instead, the hardenable resin-coated proppant flows over the uncoated proppant. This results in unconsolidated proppant at the bottom of the fractures adjacent to the wellbore. During the subsequent production of formation fluids through the propped fractures, the unconsolidated proppant at the bottom of the fracture flows back with the formation fluids. The flow-back of the proppant with the formation fluids may erode metal equipment, plug piping and vessels, and cause damage to valves, instruments and other production equipment.
Another problem encountered in the use of prior hardenable resin compositions for coating proppant particles is that the hardenable resin composition components, i.e., the liquid hardenable resin component and the liquid hardening agent component, have heretofore had very short shelf lives. That is, the shelf lives of the hardenable resin composition components once batch mixed have heretofore been as short as about four hours or less. In addition, the hardenable resin composition components have heretofore had very low flash points, i.e., flash points of about 60° F. or below, making them dangerous to use. Heretofore, it has been a common practice to utilize proppant that is precoated with a resin composition and stored. When the precoated proppant is required for use at a job site, it is transported to the job site. When such precoated resin-coated proppant is consolidated after being placed by a hardening agent, the resulting consolidated proppant pack often does not have enough strength to prevent deterioration and proppant flow-back.